Because it isn't an official release and has not been tested as exhaustively. That's the point of releases. Just take a look at the TCEC what happened to Komodo when going with a dev version.

Besides, the release has been rated in many games while the "dev version of today" has not.

It is still unsatisfying to use that old of a SF.

Judging from the score of the match it wouldn't matter if the latest SF dev were used, AlphaZero should dominate it easily. And that was with just 4 hours of training. Imagine how much stronger AlphaZero is now.